Seals Hold Vacuum in Dryer

McIntosh, Alabama is home to Huntsman, a global manufacturer and marketer of differentiated chemicals. Its operating companies manufacture products for a variety of global industries. Their plant here manufactures specialty products; one of which is used in the electronics industry and another used in the aerospace industry.

Both of these products are formed as abrasive pastes and then go through a drying process. Product A starts out as a mixture of solids, water and solvent. It is dried in a horizontal rotary vacuum dryer above 150º C. and ends up as a fluffy powder. Product B starts as a different mixture of solids, water and a solvent. It is dried in the same dryer above 50º C; the final product is a packable powder.

The rotary vacuum dryer used at the Huntsman plant has a horizontal, hollow shaft through which steam passes, via a rotary union joint at one end. The 10.5”-diameter shaft rotates within a jacketed outer shell; the jacket contains heated oil. The dryer is 5′ in diameter and 20′ long; it operates at 70-80% of its volume capacity. Pressure in the dryer ranged from 14 psig to 20mm Hg vacuum.

The dryer shafts were sealed with packed glands at either end; at first the team used graphite packing, then they tried PTFE. Neither material was able to hold vacuum in the dryer. When the stainless steel shaft is heated from room temperature to operating temperature, the shaft’s diameter grows by up to 1/4”. This compresses and strains the packing inside the stuffing boxes and creates space around the shaft as it cools at the end of a batch cycle. The packing “remembers” this compressed shape and the space created allows air into the process.

Maintenance crews were replacing the packings every 3 months or so; even when new packings were installed, the dryer still did not pull full vacuum. The length of process time that it took to dry Product A was 60 hours per batch and to dry Product B was 12 hours per batch under these conditions.

Due to the abrasive nature of the product in the dryer, it would tend to erode the shaft slowly when it migrated into the packing glands. Maintenance management knew that if they did not find a better seal, they would have to replace the shaft in the machine. Abrasion of the packing itself also caused issues with product purity. Huntsman could not allow particles of packing or grease of any kind to enter its product.

One other problem the company experienced with the packing glands was that nitrogen was leaking into the dryer around the seals, cooling the end plates somewhat. This caused condensation to build up on the end plates; product then stuck to the condensation and built up as flakes on them. It became part of every batch change to scrape this flaky buildup off the end plates inside the dryer.

Huntsman’s Production Manager, Alan Curtis, saw a seal ad in a trade magazine and contacted the company’s distributor, Jerry and Bernie Chevalier of Mid South Mechanical Sealing. Jerry helped the team to measure the shaft and gather other necessary data to design custom-engineered seals to fit the dryer.

The seal design chosen by MECO engineers for this application is an “EX-PAC” model, which has no elastomer and instead a mechanical drive mechanism. No elastomer could be used in the seal because no elastomeric material would have been compatible with both of the solvents used in the two processes. The dual seal faces are loaded by external spring force. The seals are capable of accommodating thermal growth of the shaft as it changes temperature, so they contain the vacuum in the dryer effectively. The EX-PAC seal is designed for use in applications where aggressive chemistries, abrasive product, high-temperatures or pressure fluctuations exist. Huntsman’s dryer had all of these conditions.

Due to the degree of shaft wear, an adapter plate was supplied to extend the seal from the shaft.

The seals were installed in June of 2002. The Chevalier brothers were there to assist the maintenance crew to install the MECO seals on the dryer. It took one day to install both seals because they were built split and the bearings did not need to be removed.

Since installation of the MECO seals on the vacuum dryer, the batch cycle time for Product A has been reduced to 48 hours, a 20% reduction. The cycle time for Product B was reduced from 12 to about 9.5 hours. The shorter drying time is achieved due to the seals’ ability to pull vacuum.

The MECO seals have been rebuilt twice since being installed in 2002. Rebuilding these seals consists of replacing the two rotors, which are sacrificial wear parts, when a certain amount of axial wear occurs. This is determined by monitoring a pressure gauge on the purge line. Seal purge pressure is to be kept at 2 psi above the pressure in the dryer. When the pressure drops below this level, it indicates that the rotors have worn beyond the point where the springs can maintain adequate seal face pressure. Purge gas consumption with MECO seals is typically .25 +/-.25 cfm.

Production Engineer Mitch Halstead reports that the problem with flaky product buildup is all but gone now. Operators scrape off the end plates about once in every 16 batches.

According to Steve Goldman, Huntsman’s Maintenance Facilitator, the chief benefits of switching to MECO seals were shorter processing time, reduced downtime for rebuilds and the avoided cost of re-shafting the dryer.

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